An important distinction of wireless telecommunications over wireline telecommunications is that the user of the wireless terminal is afforded the opportunity to use his or her terminal anywhere. A disadvantage of wireless telecommunications lies in that fact that because the user is mobile, an interested party might not be able to readily ascertain the location of the user.
Such interested parties might include both the user of the wireless terminal and a remote party. There are a variety of reasons, not discussed here, why the user of a wireless terminal might be interested in knowing his or her location. In various scenarios, however, it is critical that a remote party know the location of the user. In a first scenario, for example, the recipient of an E 9-1-1 emergency call from a wireless terminal might be interested in knowing the location of the wireless terminal so that emergency services vehicles can be dispatched to that location. In a second scenario, a law enforcement agency or some other organization might be conducting surveillance on the user of the wireless terminal, by tracking the location of the wireless terminal over time. Notably, in certain situations the user might not necessary want to be tracked, such as when the user is engaged in illegal activity including drug trafficking, kidnapping, and terrorism.
There are many techniques in the prior art for estimating the location of a wireless terminal. In accordance with some techniques, the location of a wireless terminal is estimated, at least in part, from measurements that are reported by the wireless terminal. The reported measurements are of radio signals measured by the wireless terminal that are transmitted by one or more base stations and, in some cases, by Global Positioning System (GPS) satellites. Some techniques rely on signal-strength measurements, while some other techniques rely on time-based measurements, while still some other techniques rely on other types of measurements. In order for these estimation techniques to work, at least some of the transmitted signals have to be strong enough to allow for accurate measurement by the wireless terminal and for reliable processing by the particular technique.
Not surprisingly, when wireless coverage is spotty or nonexistent in a geographic region, such as in a sparsely populated or infrequently travelled area, at least some location estimation techniques in the prior art are incapable of providing a reliable location estimate of a wireless terminal, much less a series of such estimates. Consequently, tracking systems that rely on such location estimates encounter problems with providing reliable surveillance of wireless terminal users.